Imagine that out of every 10 people who regularly read LW, only 3 clicked through to the original article, and out of those 3, only 1 filled out the survey.
That's two sequential filters. Now supposing that there's a bias where people with high AQ scores are both more likely to click through to the article, and once they're on the page more likely to bother to click through to the survey is reasonable. However, the data seems to rule that out.
The only hypothesis that still remains is that there is a big bias for clicking through to the article, but no bias at all for actually taking the (considerable) time to fill out the survey.
This seems somewhat contrived to me. Why would we expect that?
That is exactly what Psychohistorian expected.
Also, we don't know that there was "no bias at all" for taking the survey, just that the net change in bias between the first group and the second group was relatively small. I expected there to be a big bias in who made it through the first filter (reading the original article and finding out about the poll in its last paragraph) and multiple additional biases which would partially cancel out, and predicted that the net effect of these additional biases would be for the second group's AQ to be a bit ...
Followup to: Aspergers Poll results
Since my little survey about the degree to which the Less Wrong community has a preponderance of people with systematizing personality types, I've been collecting responses only from those people who considered taking the survey after looking at the original post, but didn't, in order to combat nonresponse bias.
82 people responded to the initial survey, and another 186 responded after the request for non-responders to respond. In the initial survey, 26% of responders scored 32+ (which is considered to be a "high" score, and out of a group of Cambridge mathematics students, 7 out of 11 who scored over 32 were said to fit the full diagnostic criteria for aspergers syndrome after being interviewed).
In the combined survey of 82 initial responders and 186 "second"-responders, this increased to 28%. In the original survey, 5% of respondents said they had already been diagnosed with aspergers syndrome, and in the combined survey this increased to 7.5%.
Overall, this indicates that response bias is probably not significantly skewing our picture of the LW audience, though, as always, it is possible that there is a more sophisticated bias at work and that these 268 people are not representative of LW.